In image forming apparatuses such as digital copiers and the like using electrophotographic methods, analog image data of a document read by a charge-coupled device (CCD) sensor are converted to digital data and subjected to image quality correction processing including modulation transfer function (MTF) correction, halftone correction and the like. Thereafter, the digital data are temporarily stored in a memory unit. Then, the digital data are read out from the memory unit as needed, subjected to editing processes of enlargement, reduction and the like, and subsequently again converted to analog data to generate laser diode (LD) drive signals based on the data so as to produce the emission of a laser beam from a semiconductor laser.
In image forming apparatuses executing complex data processing such as described above, the circuits of the apparatus are necessarily large and many circuit boards are required. Therefore, a disadvantage arises in that if an error condition occurs in such an apparatus, it is difficult to ascertain the point of origin of the problem, i.e. as being due to erroneous data read from the memory unit, or another problem.
It has been suggested, for example, that an apparatus should generate a checksum for each image unit when image data are transmitted or stored in memory, and the checksums should be stored together with the image data. Since a checksum is used to verify that image data are identical to the state when the image data were generated, a checksum is unsuitable for systems such as the aforesaid image forming apparatus of a digital copier inasmuch as the data input to the memory unit differs from the actual data output due to the various types of image processing to which they are subjected. Image processing in this instance includes, for example, variable magnification, rotation, shift, 2-in-1 and the like.